1. Field of the Invention
The invention relates generally to plant biotechnology. More specifically, the invention relates to methods and compositions for improving the efficiency of bacterially-mediated plant transformation.
2. Description of Related Art
During natural Agrobacterium-mediated transformation of plant cells, a piece of DNA from the Ti plasmid of A. tumefaciens or Ri plasmid of A. rhizogenes is transferred into the plant cell (e.g. Gelvin, 2003). This transferred DNA (T-DNA) fragment is flanked by imperfect 24 bp direct repeats that are recognized by Agrobacterium endonuclease VirD2 to produce a single stranded T-strand by nicking at a specific site in one strand of each repeat. The repeat that initiates formation of single stranded T-strand has been termed the “right border” (RB), while the repeat terminating formation of single-stranded T-DNA has been termed the “left border” (LB). The VirD2 protein is attached to the 5′ end of the strand after nicking, and guides the T-strand into plant cells where the T-strand is integrated into the plant genome with the help of other Agrobacterium and plant-encoded proteins. Sequences downstream (in a 5′ to 3′ direction) of the T-DNA region, including vector backbone sequence, may be transferred as well (e.g. Kononov et al., 1997). This likely occurs by inefficient nicking of at least one of the borders in Agrobacterium prior to transfer to a plant cell.
Comparison of the RB and LB sequences from a variety of Agrobacterium strains indicated that both RB and LB share a consensus motif (Canaday et al., 1992), which indicates that other elements may be involved in modulating the efficiency of RB processing. Cis-acting sequences next to the RB are present in many Agrobacterium strains, including A. tumefaciens and A. rhizogenes. These sequences are necessary for wild type virulence (Veluthambi et al., 1988; Shurvinton and Ream, 1991; Toro et al., 1989; Hansen et al., 1992). The sequence in A. tumefaciens was called an “overdrive” or “T-DNA transmission enhancer” by Peralta et al., (1986). In A. rhizogenes the sequence has been termed the “T-DNA transfer stimulator sequence” (TSS″) by Hansen et al (1992). The overdrive (“OD”) sequence was initially defined as a particular 24 bp motif present immediately in front of the RB repeat of octopine Ti TL-DNA (Peralta et al., 1986). A similar sequence is present in from of the RB repeat of octopine Ti TR-DNA and also in front of nopaline Ti RB and agropine Ri TL right border (Peralta et al., 1986, Shaw et al., 1984, Barker et al., 1983, Slightom et al., 1985. Further comparison of different A. tumefaciens strains revealed a 8 bp overdrive core sequence present in front of all right border sequences including nopaline strain pTiT37, octopine strain pTiA6 and A. rhizogenes pRiA4 (Peralta et al., 1986).
The presence of octopine overdrive sequence enhanced single strand T-DNA formation in Agrobacterium cells and improved T-DNA transfer into plant cells, and was necessary for wild type virulence (Peralta et al., 1986, Shurvinton and Ream 1991). The LB repeat from nopaline-producing Ti plasmid pTiT37 is capable of producing single-stranded T-DNA with high efficiency when the pTiT37 RB proximal cis-acting sequence was placed in front of it, indicating that an overdrive-like sequence indeed is also present on a nopaline Ti plasmid (Culianez-Macia and Hepburn 1988, Peralta et al., 1986), just as it is in the other identified (octopine-producing) Ti plasmids. Integration of a heterologous octopine overdrive sequence in front of nopaline pTiT37 RB resulted in much greater virulence than the parental strain which contained only a synthetic pTiT37 RB repeat (Peralta et al., 1986).
The VirC1 protein binds to overdrive and is thought to improve VirD2 nicking (Toro et al., 1988, 1989), while mutation of virC results in attenuated virulence in plants (Close et al., 1987) and reduced production of processed single stranded T-DNA sequence. Both A. tumefaciens octopine and nopaline Ti plasmids contain virC and can complement the virC mutation in trans to restore the attenuated virulence to wild type level (Close et al., 1987).
The TSS found in A. rhizogenes strains 8196, A4 and 2659 plays a similar role as the overdrive sequence in A. tumefaciens. Each A. rhizogenes strain has a different but related sequence (Hansen et al., 1992). The 8 bp TSS core sequence repeats 5 times in pRiA4, 6 times in pRi8196 and 17 times (rather than 16× as Hansen et al., 1992) in pRi2659 (Genbank accession AJ271050). pRiA4 has a conserved 8 bp overdrive core sequence in addition to the repeats. Shorter core sequence repeats in pRiA4 and pRi8196 were not sufficient for wild type virulence (Hansen et al., 1992).
Depicker et al. (U.S. Patent Publication 2003/0140376, and corresponding international publication WO01/44482) describe recombinant constructs with modified T-DNA borders in order to lessen or prevent transference of vector backbone sequences. Conner et al., (WO 05/121346) describe creation and use of sequences from T-DNA border-like regions that comprise sequences derived from plants. Heim et al. (U.S. Publ. 2003/0188345) describe vectors for Agrobacterium-mediated transformation of plants with modified border regions. Lassner et al., (U.S. Publ. 2006/0041956) describe modifications to T-DNA border regions to enable identification of transgenic events that do not comprise non T-DNA sequences.
While the foregoing studies have increased understanding in the art, what remains needed is a method to improve the efficiency of Agrobacterium-mediated plant transformation. Although the presence of overdrive or TSS sequences increases virulence of wild type Agrobacterium and improves T-DNA transfer into plant cells compared to plasmids lacking the sequences, it has remained unclear how to further improve transformation efficiency including through the use of overdrive or TSS sequences.